1. Field of the Invention
The present invention relates to a method of growing in vapor phase a semiconductor crystal film consisting of a nitrogen compound and, more particularly, to a method of vapor-growing a semiconductor crystal film by blowing a reaction gas on the surface of a substrate.
2. Description of the Related Art
Generally, GaN, InN, AlN, and their semiconductor crystals are vapor-grown by apparatuses shown in FIGS. 1 to 4.
For example, a method of growing GaN on the surface of a substrate will be described below with reference to FIG. 1.
i) A sapphire with (0001) orientation (C face) is generally used as a substrate. A sapphire substrate 1 is placed on a carbon susceptor 4 in a chamber 6.
ii) While H.sub.2 is flowed, the carbon susceptor 4 is heated up to a high temperature of 950.degree. C. to 1,150.degree. C. by radio frequency induction heating means 7.
ill) Trimethylgallium (TMG) as a Ga source, ammonia (NH.sub.3) as an N source, and H.sub.2 as a carrier are blown to the surface of the substrate from a reaction gas blow tube.
These reaction gases are blown to a portion above and very close to the substrate 1 from a thin reaction gas blow tube 2 having an inner diameter of about 5 mm to 10 mm. The reaction gases are blown against the substrate as a high-speed flow at a flow rate of 2 m/sec or more.
In this manner, a GaN film having a thickness of about 2 to 5 .mu.m can be grown on the sapphire substrate in a growth time of 30 to 60 minutes.
FIG. 2 shows a conventional semiconductor crystal film growth apparatus. As shown in FIG. 2, this apparatus comprises a chamber, a carbon susceptor 43 which is non-rotating and incorporated in the chamber and on which a substrate can be set obliquely to the horizontal direction, a reaction gas blow tube 45 capable of horizontally blowing a reaction gas to the substrate, and a reduction gas H.sub.2 blow tube 44. A RF coil 42 is horizontally arranged and wound around the outer circumferential wall of the chamber around the carbon susceptor 43. A load-lock chamber 41 is connected to the chamber. A rotary pump 46 is connected to the chamber, and a turbo pump 46 47 and a rotary pump 48 are connected to the load-lock chamber 41. In this apparatus, a reaction gas is horizontally blown to a substrate set obliquely to the horizontal direction on the fixed carbon susceptor. Note that the carbon susceptor is not rotated and no pressing gas is used.
FIG. 3 shows an apparatus almost similar to the apparatus shown in FIG. 1. This apparatus comprises a chamber 51, a rotary carbon susceptor 53 incorporated in the chamber 51, a substrate 59 set on the susceptor 53, a reaction gas blow tube 55 extending downward from the upper portion of the chamber 51, and a reduction gas H.sub.2 blow tube 54. A vertically RF coil 52 is arranged and wound around the outer circumferential wall of the chamber 51 around the carbon susceptor 53.
FIG. 4 shows an apparatus comprising a chamber 61, a rotary carbon susceptor 63 which is incorporated in the chamber 61 and on which a substrate 69 can be set obliquely to the vertical direction, and a reaction gas blow tube 65 extending downward from the upper portion of the chamber 61. The reaction gas blow tube 65 is branched into lines 70 and 71 at its upper portion. A gas mixture of NH.sub.3 and H.sub.2 is flowed through the line 70, and a gas mixture of TMG and H.sub.2 is flowed through the line 71. A vertically RF coil 62 is arranged and wound around the outer circumferential wall of the chamber 61 around the susceptor 63.
In order to grow a semiconductor crystal film on a substrate by the above method, the flow rate of a reaction gas must be increased. That is, no GaN can be grown unless the flow rate of a reaction gas is 2 m/sec or more. This is assumed to be attributed to the fact that if the flow rate of a reaction gas is not sufficiently high, the reaction gas is prevented from reaching the substrate by a large heat convection current formed by a high reaction temperature.
For this reason, in order to obtain a high-speed flow of a reaction gas, a thin reaction gas blow tube having an inner diameter of 5 to 10 mm is used in conventional growth methods. The lower opening end of this thin reaction gas blow tube is located at a position separated from above the surface of a substrate by 5 to 10 mm.
When GaN is grown on the surface of a sapphire substrate in this state, the diameter of an area of the grown semiconductor crystal film is about 5 to 10 mm, i.e., only a very small film can be obtained. For example, when a sapphire substrate having a diameter of 2 inches is used, the area of a grown semiconductor crystal film is about 2/50 or less the area of the substrate, resulting in very low yield. That is, the conventional growth methods cannot uniformly grow a semiconductor crystal film in a large area having a diameter of 10 mm or more on the surface of a substrate.
In addition, in the conventional methods, a large amount of a GaN reaction product adheres on the distal end of the thin reaction gas blow tube each time a semiconductor crystal film is grown on the surface of a substrate. If the temperature of the substrate is increased during the growth of the semiconductor crystal film, GaN deposited on the reaction gas blow tube is decomposed into Ga metal by the influence of this heat, and the Ga metal falls on the substrate during the reaction. Since no GaN is grown on portions where Ga fell, the growth yield is extremely decreased.
In an extreme case, the thin reaction gas blow tube must be replaced by a new one or washed each time the reaction is caused, resulting in very poor operability.
When a CVD method such as a metal organic chemical vapor deposition method (to be referred to as an MOCVD method hereinafter) or a molecular beam epitaxy method is used to grow semiconductor crystals on a substrate set in a reactor, it is important to prepare a "monitor window" free from contamination in the reactor. During the growth of semiconductor crystals, this "monitor window" is used to externally observe the crystal growth state, or light is radiated on a crystal layer through the "monitor window" to give optical energy to the growing crystal layer, thereby changing the growth state.
Conventionally, several structures have been proposed and used as the "monitor window" of a reactor. FIG. 5 shows a conventional apparatus using an example of the "monitor window". This reactor vessel 10 has a double cylinder structure using quartz tubes. An induction coil 12 for heating a substrate 18 is wound outside an outer tube. A "monitor window" 13 is open in an inner tube at a position above the substrate 18. A reaction gas is supplied into the inner tube to grow a semiconductor crystal layer on the surface of a substrate. Hydrogen, for example, is flowed into the outer tube to prevent the reaction gas from flowing into the outer tube.
In the apparatus having the above structure, however, since the inner surfaces of the inner and outer tubes are contaminated by the reaction gas, these tubes cannot be successively used a plurality of times. That is, since the reaction gas constantly flows through the inner tube, a decomposition product of the reaction gas immediately adheres to blacken the inner surface of the inner tube. In the outer tube, similarly, hydrogen gas flowing through the outer tube forms a turbulent flow near the "monitor window" to allow the reaction gas to flow into the outer tube. The reaction gas is brought into contact with the inner surface of the outer tube at a portion located above the "monitor window" and gradually contaminates this portion to make it difficult to observe a substrate.
Therefore, the entire reactor must be replaced after it is used only several times. In the replacement of the reactor, since moisture in air is adsorbed in the interior of the reactor, complicated steps of, e.g., vacuum baking or gas baking must be repeatedly performed a number of times over several days in order to remove the moisture, resulting in a very cumbersome operation.
As a method of growing a semiconductor crystal layer, a method of growing crystals while radiating light is available. In this method, a growth temperature can be significantly decreased by radiating light, and a light source 14 is arranged outside a reactor in order to radiate light. The light source radiates an epitaxial crystal layer with light which is transmitted through a "monitor window". In this method, it is very important to keep the "monitor window" clean because a contaminated "monitor window" absorbs light to reduce light radiation intensity on the epitaxial crystal layer.